Although ultra-violet radiation has long been recognized as a factor in the development of cutaneous cancer, aging of the skin, and mutagenic changes, it is only within the last decade or less that ultra-violet radiation has been universally recognized as a causative factor in ocular pathogenesis.
In humans, the eye has evolved into a sophisticated organ having neurophysiologic responses to photons in a certain portion of the electromagnetic spectrum, that provides a constant detailed map of the immediate environment. The action spectrum for these responses lie primarily within the 400-700 NM wavelength range, which has been labeled the visible spectrum or "Light".
The maximum of the eye's spectral response corresponds roughly to the maximum of solar spectral irradiance. Because solar UV radiation is present during most of the daylight hours, the eye may be exposed daily to some amount of solar ultraviolet radiation throughout life.
Wavelengths shorter than approximately 290NM or UV-C are partially or completely absorbed within the cornea and conjunctiva. The acute effects of exposure to these wavelengths are primarily those of conjunctivitis and a corneal inflammation reaction known as photokeratitis. The inflammatory reaction of the outermost layer of the eye to UV-C and UV-B radiation can be similar to that of the skin in some respects.
The clinical progress or picture of photokeratitis follows a characteristic course. For example, after exposure, there is a period of latency which varies somewhat inversely with the amount of exposure. The latent period may be as short as 30 minutes or as long as 24 hours but it is typically 6 to 12 hours.
Conjunctivitis, which is often accompained by an erythema of the skin surrounding the eyelids, is associated with the sensation of a foreign body or "sand" in the eyes, varying degrees of photophobia (intolerance to light), lacrimation (tearing), and blepharospasm (spasm of lid muscles). Corneal pain can be very severe, and the individual is usually incapacitated for some period of time. These acute symptoms usually last from 6 to 24 hours and almost all discomfort disappears within 48 hours. Very rarely does conjunctivitus causing exposure result in permanent damage.
However, unlike the skin, the ocular system does not develop tolerance to repeated ultraviolet exposure. Swelling or shrinking of groups of corneal epithelial cells leads to visibly recongizable stippling or irregular mosaic granulation of the corneal surface. With UV doses greater than the threshold for photokeratitis, surface epithelial cells show nuclear fragmentation, mid-epithelial cells show vacuole formation, and basel cells show inhibition of mitosis and clouding of the corneal stroma occurs. Inflammation is also present in the conjunctiva where vasodilation, edema, and inflammatory cell infiltrate is followed by desquamation.
When the cornea is damaged or replaced by surgery, the eye in general becomes very sensitive and the retina becomes vulnerable to ultra violet radiation below about 325 nm or in the UV-B and UV-C ranges. This is the opposite of what happens with a healthy or non-relaced cornea. In the healthy or undamaged eye, absorption of radiation by the cornea and lens of the human eye is such that very little radiation of wavelengths shorter than 390 NM reaches the retina.
The retina (tunica interna) is a delicate nervous membrane, upon which the images of external objects are received. Its outer surface is in contact with the choroid; its inner with the vitreous body. Behind, it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward and extends nearly as far as the ciliary body, where it ends in a jagged margin, the ora serrata. Here the nervous tissues of the retina end, but a thin prolongation of the membrane extends forward over the back of the ciliary processes and iris, forming the pars ciliatis retinae and pars inidica retinae. This forward prolongation consists of the pigmentary layer of the retina together with a stratum of columnar epithelium. The retina is soft, semitransparent, and of a purple tint in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; however, it soon becomes clouded, opaque, and bleached when exposed to sunlight. Prolonged exposure of the retina to UV-B and UV-C wavelengths causes damage to the retina.